1. Field of the Invention
One embodiment of the present invention relates to a storage device, a semiconductor device, an electronic device, and a server system.
Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. The present invention relates to a process, a machine, manufacture, or a composition of matter. One embodiment of the present invention particularly relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a storage device, a driving method thereof, or a manufacturing method thereof.
In this specification and the like, a semiconductor device refers to an element, a circuit, a device, or the like that can function by utilizing semiconductor characteristics. An example of the semiconductor device is a semiconductor element such as a transistor or a diode. Another example of the semiconductor device is a circuit including a semiconductor element. Another example of the semiconductor device is a device provided with a circuit including a semiconductor element.
2. Description of the Related Art
As miniaturization of a semiconductor device progresses, a problem of a soft error due to radiation or the like becomes more apparent. Unlike a hard error, it is difficult in many cases to determine a cause of a soft error because data is updated and restored to a correct state by a restart or the like even after the occurrence of the soft error. As described above, a soft error that occurs in a logic circuit or a storage device may cause malfunction of a processor, a control digital circuit, or the like of a computer; therefore, a system that does not malfunction is needed.
A method using a cyclic redundancy check is generally known as a method for detecting an error in the transfer of digital data (see Patent Document 1). However, an error cannot be sufficiently prevented even though an error can be detected. Furthermore, a neutron ray, which is a main cause of a soft error, is difficult to block, and a countermeasure against this is difficult to take.
As portable electronic devices become widely used, the probability of occurrence of soft errors drastically varies. For example, the travel time from a basement of a tall building that is shielded by concrete (where the amount of neutron rays or α rays is small) to an airplane that flies at an ultrahigh altitude or a space station (where the amount of neutron rays or α rays is large) while the electronic devices are in operation becomes short. Therefore, a countermeasure against a soft error which is independent of use environment needs to be taken.
In addition, a countermeasure against system instability due to a soft error needs to be taken for not only devices that need to be highly reliable, such as an aircraft, a medical device, a database server, a high-capacity router, and a supercomputer, but also devices that handle a high density of information, such as an advanced electronic vehicle and a digital home appliance.
A DRAM or an SRAM, which is a conventional semiconductor memory device, is not preferable because data is lost when power supply is turned off. An EEPROM or a flash memory maintains data by storing electric charge in a so-called floating gate provided between a channel and a gate.
However, in a conventional EEPROM or flash memory, high voltage is needed at the time of injection of charge to a floating gate or removal of the charge. Because of this, deterioration of a gate insulating film cannot be avoided and it is not possible to limitlessly repeat write and erase operations. In view of this, Patent Document 2 discloses a memory configuration capable of retaining data even when power supply is stopped, in which a transistor whose semiconductor layer contains a metal oxide that is known to enable a transistor to have low off-state current.
A transistor including a metal oxide in a channel formation region (hereinafter, such a transistor may be referred to as a metal oxide semiconductor transistor or an OS transistor) is known. For example, a storage device in which a write transistor of a gain cell is an OS transistor is disclosed in Patent Document 3. In the gain cell, the amount of accumulated electric charge can be amplified by a read transistor and the electric charge can be supplied to a bit line; therefore, it is possible to reduce the capacitance of the capacitor.